What is the true position for Upper Rail to set the canon ef 18mm 1:2.8?
Unfortunately we don't have the settings for this lens.
As we hear back from customers and learn settings we post the findings here: http://www.nodalninja.com/html/settings.html
If you do not find your camera/lens on the list the manual teaches you how to find these settings: http://nodalninja.com/nodal_ninja_user_manual.html.
I can tell you the lower "A" setting once found will never change.
It really is easy to learn how to find the no parallax point of a lens and we really encourage our customers to learn this. Once learned it will work for any camera/lens at any focal length.
Also here is another link that you might find helpful: http://www.johnhpanos.com/epcalib.htm
After you do finally learn the settings I hope you might share them with us so we can post them on the website.
Hope this helps.